Pixel circuit, organic electro-luminescent display apparatus, and method of driving the same

ABSTRACT

A pixel circuit includes a light emitting device, an N-type driving transistor for outputting a driving current according to a voltage applied to the gate electrode, a first capacitor coupled to a gate electrode of the driving transistor, a second capacitor including a first terminal coupled to the gate electrode of the driving transistor and a second terminal coupled to the first electrode of the light emitting device, and second through sixth N-type transistors. An initialization period is reduced for a driving operation and the threshold voltage compensation time of a driving transistor is controlled.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2010-0000188, filed on Jan. 4, 2010, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND

1. Field

Aspects of embodiments according to the present invention relate to apixel circuit implemented using N-type transistors, an organicelectro-luminescent display apparatus, and a method of driving the same.

2. Description of the Related Art

One type of flat panel display apparatuses is an organicelectro-luminescent display which displays images by using organic lightemitting diodes (OLEDs) that emit light by the recombination ofelectrons and holes. The organic electro-luminescent display apparatushas high response speed and low power consumption. The organicelectro-luminescent display apparatus applies a data driving signalaccording to input data to a plurality of pixel circuits. The datadriving signal controls the brightness of each pixel and thereby acorresponding image to a user is provided.

SUMMARY

Accordingly, an aspects of the present invention provides a sufficientinitialization time and a sufficient threshold voltage compensation timefor a driving transistor when implementing a pixel circuit and anorganic electro-luminescent display apparatus by using N-typetransistors.

According to an embodiment of the present invention, a pixel circuitincludes: a light emitting device including a first electrode and asecond electrode; a driving transistor including: a first electrode; asecond electrode; and a gate electrode, the driving transistor foroutputting a driving current according to a voltage applied to the gateelectrode of the driving transistor; a first capacitor including a firstterminal and a second terminal, the first capacitor being coupled to thegate electrode of the driving transistor; a second capacitor including afirst terminal coupled to the gate electrode of the driving transistorand a second terminal coupled to the first electrode of the lightemitting device; a second transistor including: a first electrodecoupled to the gate electrode of the driving transistor; a secondelectrode coupled to the first electrode of the driving transistor; anda gate electrode; a third transistor for applying a first power supplyvoltage to the first electrode of the driving transistor in response toan emission control signal; a fourth transistor including a firstelectrode for receiving a data signal and a second electrode coupled tothe first terminal of the first capacitor; a fifth transistor including:a first electrode for receiving a reference voltage; a second electrodecoupled to the first terminal of the first capacitor; and a gateelectrode; and a sixth transistor including: a first electrode forreceiving a first voltage; a second electrode coupled to the gateelectrode of the driving transistor; and a gate electrode, wherein thedriving transistor and the second to sixth transistors are N-typetransistors.

The sixth transistor may be configured to apply the first power supplyvoltage to the gate electrode of the driving transistor in response to afirst scan signal applied to the gate electrode of the sixth transistor.

The fifth transistor may be configured to apply the reference voltage tothe first terminal of the first capacitor in response to a second scansignal applied to the gate electrode of the fifth transistor, and thesecond transistor is configured to diode-connect the driving transistorin response to the second scan signal applied to the gate electrode ofthe second transistor.

The second scan signal may be a scan signal of an n^(th) period.

The first scan signal and the second scan signal may both be maintainedat a high signal level during an overlap period.

The second scan signal may be maintained at a high signal level for aduration greater than the overlap period.

The first scan signal may be a scan signal of an (n−2)^(th) period.

The fourth transistor may transfer the data signal to the first terminalof the first capacitor in response to a third scan signal applied to thegate electrode of the fourth transistor.

The third scan signal may be a scan signal of an (n+3)^(th) period.

The light emitting device may be an organic light emitting diode (OLED).

The driving transistor and the second to sixth transistors may be N-typemetal-oxide semiconductor field effect transistors (MOSFETs).

The first electrode of the driving transistor may be a drain electrodeand the second electrode of the driving transistor may be a sourceelectrode.

The first voltage applied to the first electrode of the sixth transistormay be the first power supply voltage.

The first voltage applied to the first electrode of the sixth transistormay be an initial voltage.

The first voltage applied to the first electrode of the sixth transistormay be the reference voltage.

According to another embodiment of the present invention, a method ofdriving a pixel circuit including an organic light emitting diode(OLED), a driving transistor, a plurality of switching transistorsconfigured to turned on in response to first to third scan signals andan emission control signal, a plurality of storage capacitors, and aboosting capacitor coupled between first electrodes of boostingtransistors, wherein the driving transistor, the switching transistors,and the boosting transistors are NMOS transistors, the method including:initializing the pixel circuit when the first scan signal has a highsignal level; diode-connecting the driving transistor to compensate fora threshold voltage of the driving transistor when the second scansignal has the high signal level; writing a data signal in the pixelcircuit when the third scan signal has the high signal level; andflowing a current through the OLED to emit light in response to thewritten data signal when the emission control signal has the high signallevel.

The first scan signal and the second scan signal may both be maintainedat a high signal level during an overlap period.

The second scan signal may be maintained at a high signal level for aduration greater than the overlap period.

According to another embodiment of the present invention, an organicelectro-luminescent display apparatus including: a scan driver forsupplying a scan signal to a plurality of scan lines; an emissioncontrol driver for supplying an emission control signal to a pluralityof emission control lines; a data driver for supplying a data signal toa plurality of data lines; and a plurality of pixel circuits located atcrossing regions of the scan lines, the emission control lines and thedata lines, wherein each pixel circuit of the plurality of pixelcircuits includes: an organic light emitting diode (OLED) including afirst electrode and a second electrode; a driving transistor including:a first electrode; a second electrode; and a gate electrode, the drivingtransistor for outputting a driving current according to a voltageapplied to the gate electrode of the driving transistor; a firstcapacitor including a first terminal and a second terminal coupled tothe gate electrode of the driving transistor; a second capacitorincluding a first terminal coupled to the gate electrode of the drivingtransistor and a second terminal coupled to the first electrode of theOLED; a second transistor including: a first electrode coupled to thegate electrode of the driving transistor; a second electrode coupled tothe first electrode of the driving transistor; and a gate electrode; athird transistor for applying a first power supply voltage to the firstelectrode of the driving transistor in response to the emission controlsignal; a fourth transistor including: a first electrode coupled to thedata signal, a second electrode coupled to the first terminal of thefirst capacitor; and a gate electrode; a fifth transistor including: afirst electrode coupled to a reference voltage; a second electrodecoupled to the first terminal of the first capacitor; and a gateelectrode; and a sixth transistor including: a first electrode coupledto the first power supply voltage; a second electrode coupled to thegate electrode of the driving transistor; and a gate electrode, whereinthe driving transistor and the second to sixth transistors are N-typetransistors.

The gate electrode of the sixth transistor may be coupled to an(n−2)^(th) scan line of the scan lines and may be configured to applythe first power supply voltage to the gate electrode of the drivingtransistor in response to the scan signal applied to the gate electrodeof the sixth transistor.

The gate electrode of the fifth transistor may be coupled to an n^(th)scan line of the scan lines and may be configured to apply the referencevoltage to the first terminal of the first capacitor in response to thescan signal applied to the gate electrode of the fifth transistor.

The gate electrode of the second transistor may be coupled to a n^(th)scan line of the scan lines and may be configured to diode-connect thedriving transistor in response to the scan signal applied to the gateelectrode of the second transistor.

The gate electrode of the fourth transistor may be coupled to an(n+3)^(th) scan line of the scan lines and may be configured to transferthe data signal to the first terminal of the first capacitor in responseto the scan signal applied to the gate electrode of the fourthtransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects will become more apparent bydescribing in detail exemplary embodiments thereof with reference to theattached drawings in which:

FIG. 1 illustrates a structure of an organic light emitting diode(OLED);

FIG. 2 illustrates an exemplary pixel circuit implemented using P-typetransistors;

FIG. 3 illustrates an organic electro-luminescent display apparatusaccording to an embodiment of the present invention;

FIG. 4 is a circuit diagram illustrating an embodiment of a pixelcircuit as illustrated in FIG. 3 according to an embodiment of thepresent invention;

FIG. 5 is a timing diagram of driving signals according to an embodimentof the present invention;

FIGS. 6 to 10 illustrate an operation process of a pixel circuit of FIG.4 according to the timing diagram shown in FIG. 5;

FIGS. 11 and 12 illustrate a structure of a pixel circuit according toanother embodiment of the present invention; and

FIG. 13 is a flow chart illustrating a method of driving an organicelectro-luminescent display apparatus according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be describedmore fully with reference to the accompanying drawings. Like referencenumerals in the drawings denote like elements, and thus any redundantdescription will be omitted for conciseness.

In general, an organic electro-luminescent display apparatus emits lightby electrically exciting a fluorescent organic compound, and displays animage by driving a plurality of pixels arranged in a matrixconfiguration. An organic light emitting device included in the pixel isreferred to as an organic light emitting diode (OLED) because it hasdiode characteristics.

FIG. 1 illustrates a structure of an organic light emitting diode(OLED).

Referring to FIG. 1, an OLED has a stack structure of an anode electrodelayer Anode formed of an indium tin oxide (ITO), an organic thin film,and a cathode electrode layer Cathode formed of a metal. The organicthin film includes an emitting layer EML, an electron transport layerETL, and a hole transport layer HTL in order to improve the balance ofelectrons and holes to enhance the light emitting efficiency. Theorganic thin film may further include a hole injecting layer HIL or anelectron injecting layer EIL.

A thin film transistor may be coupled to an anode electrode of the OLED,and the OLED may be driven according to a data voltage sustained by thecapacitance of a capacitor coupled to a gate electrode of the thin filmtransistor.

FIG. 2 illustrates an exemplary pixel circuit implemented using P-typetransistors.

Referring to FIG. 2, a switching transistor M2 is turned on by a selectsignal of a select scan line Sn, a data voltage is transferred from adata line Dm to a gate terminal of a driving transistor M1, and apotential difference between the data voltage and a voltage of a firstpower supply voltage source ELVDD is stored in a capacitor C1 which iscoupled between a source electrode of the driving transistor M1 and thegate terminal of the driving transistor M1. Due to the potentialdifference, a driving current Ioled flows through an OLED to cause theOLED to emit light. Herein, a gray level (may also be referred to as agray scale or a gray scale level) may be displayed according to thelevel of the data voltage applied.

However, the driving transistors M1 of pixel circuits may have differentthreshold voltages. If the driving transistors M1 of the pixel circuitshave different threshold voltages, then the output current levels of thedriving transistors M1 of the pixel circuits become different, thusmaking it very difficult to implement a uniform image quality. Such athreshold voltage difference between the driving transistors M1 mayincrease with an increase in the size of an organic electro-luminescentdisplay apparatus, which may degrade the image quality of the organicelectro-luminescent display apparatus. Thus, the threshold voltage ofthe driving transistor M1 in each pixel circuit of the organicelectro-luminescent display apparatus should be compensated to provide auniform image quality.

In the pixel circuit of FIG. 2, the switching transistor M2 and thedriving transistor M1 are configured using PMOS transistors, oneterminal of the capacitor C1 is coupled to the first power supplyvoltage source ELVDD, and the other terminal of the capacitor C1 iscoupled to a node A. One electrode of the switching transistor M2 andthe gate electrode of the driving transistor M1 are also coupled to thenode A. The source electrode of the driving transistor M1 is coupled tothe first power supply voltage source ELVDD, and a drain electrode ofthe driving transistor M1 is coupled to an anode electrode of the OLED.

In this case, the driving transistor M1 acts as a current source, thegate electrode of the driving transistor M1 receives the data voltage,and the source electrode of the driving transistor M1 receives thevoltage from the first power supply voltage source ELVDD. That is,because the source electrode of the driving transistor M1 is alwaysfixed at the voltage of the first power supply voltage source ELVDD, theemission voltage of the OLED does not affect a gate-source voltage Vgsof the driving transistor M1.

If the switching transistor M2 and the driving transistor M1 of FIG. 2were configured using N-type transistors, the capacitor C1 would becoupled between the gate electrode of the driving transistor M1 and thedrain electrode of the driving transistor M1.

In this case, the source electrode of the driving transistor M1 wouldnot be fixed at the voltage of the first power supply voltage sourceELVDD, which would become a source follower coupled to a load. Thus, thegate-source voltage Vgs of the driving transistor M1 would be affectedby a cathode power voltage ELVSS of the OLED and the emission voltage ofthe OLED.

The level of the voltage from the cathode power supply voltage sourceELVSS varies due to an IR voltage drop caused by the parasiticresistance of an interconnection transferring the cathode power supplyvoltage source ELVSS from a power supply and a voltage drop caused by acurrent flowing into each pixel. Accordingly, the voltage of the sourceelectrode in such a pixel circuit implemented using N-type transistorswould become unstable, which may destabilize the image brightness.

Also, the emission voltage of the OLED in the pixel circuit implementedusing N-type transistors would affects the gate-source voltage Vgs ofthe driving transistor M1. Thus, it would become sensitive to thevariation/degradation-dependent change and the temperature-dependentcharacteristics of the OLED.

FIG. 3 illustrates an organic electro-luminescent display apparatus 300according to an embodiment of the present invention.

Referring to FIG. 3, an organic electro-luminescent display apparatus300 according to an embodiment of the present invention includes adisplay unit 310, an emission control driver 302, a scan driver 304, adata driver 306, and a power supply 308.

The emission control driver 302, the scan driver 304, the data driver306 and the power supply 308 may be implemented in one IC chip.

The display unit 310 includes (n×m) pixel circuits P (P11, P12, . . . ,Pnm) each having an OLED (not illustrated), (n+3) scan lines extendingin the row direction to transfer scan signals S0, S1, S2, . . . , Sn+3,m data lines extending in the column direction to transfer data signalsD1, D2, . . . , Dm, and n emission control lines extending in the rowdirection to transfer emission control signals E1, E2, . . . , En.

Each of the pixel circuits P receives the scan signal, the data signal,the emission control signal, a voltage from a first power supply voltagesource ELVDD, a voltage from a second power supply voltage source ELVSS,a reference voltage from a reference voltage source Vref and an initialvoltage Vinit to cause their respective OLEDs to emit light and therebydisplay an image.

The emission control driver 302 is coupled to the emission control linesto apply the emission control signals E1, E2, . . . , En to the displayunit 310. The scan driver 304 is coupled to the scan lines to apply thescan signals S0, S1, S2, . . . , Sn+3 to the display unit 310. The datadriver 306 is coupled to the data lines to apply the data signals D1,D2, . . . , Dm to the display unit 310. Herein, the data driver 306supplies a data current to the pixel circuits P during a programmingperiod. The power supply 308 supplies the voltage of first power supplyvoltage source ELVDD, a voltage of the second power supply voltagesource ELVSS, a voltage of the reference voltage source Vref, and theinitial voltage Vinit to each of the pixel circuits P.

FIG. 4 is a circuit diagram illustrating an embodiment of the pixelcircuit as shown in FIG. 3.

FIG. 4 illustrates the pixel circuit Pnm located at the n^(th) row andm^(th) column. The pixel circuit Pnm receives the data signal Dm fromthe data driver 306 through the corresponding data line and outputs adriving current corresponding to the data signal Dm. Also, the pixelcircuit Pnm is coupled to the (n−2)^(th), n^(th) and (n+3)^(th) scanlines to receive the scan signals.

The pixel circuit Pnm according to an embodiment of the presentinvention includes a driving transistor T1, second to sixth transistorsT2, T3, T4, T5 and T6, a light emitting device, a first capacitor C1,and a second capacitor C2.

The light emitting device may be an OLED that has a similar (or thesame) structure as illustrated FIG. 1. The OLED has a first electrodecorresponding to an anode electrode and a second electrode correspondingto a cathode electrode. According to an embodiment of the presentinvention, the OLED has an anode electrode coupled to a source electrodeof the driving transistor T1 and a cathode electrode coupled to thesecond power supply voltage source ELVSS.

The first capacitor C1 has a first terminal coupled to a first node N1and a second terminal coupled at a second node N2 to a gate electrode ofthe driving transistor T1.

The second capacitor C2 has a first terminal coupled to the second nodeN2 and a second terminal coupled to the source electrode of the drivingtransistor T1.

The driving transistor T1 and the second to sixth transistors T2, T3,T4, T5 and T6 of the pixel circuit Pnm may be N-type transistors such asN-type metal-oxide semiconductor field effect transistors (MOSFETs). TheN-type transistor is turned on/off according to whether a high or lowsignal level is applied to its gate electrode. An oxide oramorphous-silicon (amorphous-Si) transistor can be implemented at alower cost than a poly-silicon (poly-Si) transistor. However, in adisplay panel using an oxide or amorphous-Si transistor as a backbone, apixel circuit should be implemented using only N-type transistorscapable of compensating for the device characteristic distribution.Thus, an embodiment of the present invention provides a pixel circuitthat is configured using only N-type transistors.

The driving transistor T1 has a first electrode D corresponding to adrain electrode and a second electrode S corresponding to a sourceelectrode and outputs a driving current according to a voltage appliedto its gate electrode.

The second transistor T2 has a first electrode coupled to the gateelectrode of the driving transistor T1 (e.g., at the second node N2) anda second electrode coupled to the drain electrode D of the drivingtransistor T1. The second transistor T2 diode-connects the drivingtransistor T1 in response to the scan signal Sn of the n^(th) scan lineapplied to its gate electrode.

The third transistor T3 has a first electrode coupled to the first powersupply voltage source ELVDD and a second electrode coupled to the drainelectrode D of the driving transistor T1. The third transistor T3applies the voltage from the first power supply voltage source ELVDD tothe drain electrode D of the driving transistor T1 in response to theemission control signal En.

The fourth transistor T4 has a first electrode coupled to the data linefor applying the data signal Dm and a second electrode coupled to thefirst node N1. Herein, the first terminal of the first capacitor C1 iscoupled to the first node N1. The fourth transistor T4 applies the datasignal Dm to the first node N1 in response to the scan signal Sn+3 ofthe (n+3)^(th) scan line.

The fifth transistor T5 has a first electrode coupled to the referencevoltage source Vref and a second electrode coupled to the first node N1.The fifth transistor T5 applies the voltage from the reference voltagesource Vref to the first node N1 in response to the scan signal Sn ofthe n^(th) scan line.

The sixth transistor T6 has a first electrode coupled to the first powersupply voltage source ELVDD and a second electrode coupled to the gateelectrode of the driving transistor T1. The second electrode of thesixth transistor T6, the gate electrode of the driving transistor T1,the second terminal of the first capacitor C1, and the first terminal ofthe second capacitor C2 are coupled to the second node N2. The sixthtransistor T6 applies the voltage from the first power supply voltagesource ELVDD to the second node N2 in response to the scan signal Sn−2of the (n−2)^(th) scan line.

According to other embodiments of the present invention, the voltage ofthe reference voltage source Vref or the initial voltage Vinit may beapplied to the first electrode of the sixth transistor T6 instead of thevoltage from the first power supply voltage source ELVDD.

FIG. 5 is a timing diagram of driving signals according to an embodimentof the present invention. FIGS. 6 to 10 illustrate an operation processof the pixel circuit of FIG. 4 according to the timing diagram shown inFIG. 5.

Referring to FIG. 5, an initialization operation is performed during aperiod (A). According to an embodiment of the present invention, thescan signal Sn−2 (i.e., an initialization signal) of the (n−2)^(th) scanline is used to provide divided initialization periods. A load on aninitialization time increases as the size of the organicelectro-luminescent apparatus increases. Therefore, if theinitialization and the compensation of the threshold voltage of thetransistor are performed concurrently (e.g., simultaneously orsubstantially simultaneously), then the time necessary for theinitialization may be decreased substantially. Thus, an embodiment ofthe present invention divides the initialization and allows for adecrease in the initialization time.

During the period (A), the scan signal Sn−2 of the (n−2)^(th) scan lineis asserted at a high signal level, and the scan signal Sn of the n^(th)scan line, the scan signal Sn+3 of the (n+3)^(th) scan line, and theemission control signal En are asserted at a low signal level.Accordingly, the sixth transistor T6 is turned on and the drivingtransistor T1 and the second to fifth transistors T2, T3 T4 and T5 areturned off.

FIG. 6 illustrates an operation of the pixel circuit in the period (A).

During the period (A), the sixth transistor T6 is turned on toinitialize the voltage of the second node N2 (i.e., the second terminalof the first capacitor C1 and the gate electrode of the drivingtransistor T1) to the voltage of the first power supply voltage sourceELVDD. During the period (A), the voltage level of the second node N2 isELVDD and the voltage level of the anode electrode of the OLED is equalto the sum (ELVSS+Vto) of the voltage of the second power supply voltagesource ELVSS and a threshold voltage Vto of the OLED.

During a period (A′), the scan signal Sn−2 of the (n−2)^(th) scan linemaintains the high signal level, the scan signal Sn+3 of the (n+3)^(th)scan line and the emission control signal En both maintain the lowsignal level, and the scan signal Sn of the n^(th) scan line changes tothe high signal level. Thus, during the high-level overlap period (A′),the scan signal Sn−2 of the (n−2)^(th) scan line and the scan signal Snof the n^(th) scan line both have a high signal level asserted.Accordingly, the second transistor T2, the fifth transistor T5 and thesixth transistor T6 are turned on, and the driving transistor T1, thethird transistor T3 and the fourth transistor T4 are turned off.

FIG. 7 illustrates an operation of the pixel circuit in the period (A′).

During the period (A′), the initialization and the compensation of athreshold voltage Vth of the driving transistor T1 are performed. Thefifth transistor T5 is turned on to initialize the first node N1 to thevoltage level of the reference voltage source Vref. That is, the firstterminal of the first capacitor C1 is initialized to the voltage levelof the reference voltage source Vref. Also, because the sixth transistorT6 is still turned on, the second terminal of the first capacitor C1 isinitialized to the voltage level of the first power supply voltagesource ELVDD.

According to an embodiment of the present invention, an overlap period(A′) occurs in which both the scan signal Sn−2 of the (n−2)^(th) scanline and the scan signal Sn of the n^(th) scan line are asserted at thehigh-level, thus allowing for a more stable initialization of the firstcapacitor C1. In the period (A′), when the second terminal of the firstcapacitor C1 is initialized to the voltage level of the first powersupply voltage source ELVDD, the first terminal of the first capacitorC1 is in a floating state. Thus, if the second terminal of the firstcapacitor C1 is not fixed and the scan signal Sn of the n^(th) scan lineapplies the reference voltage source Vref to the first terminal of thefirst capacitor C1, the voltage stored in the first capacitor C1 isaffected. Thus, the overlap period (A′) is provided to prevent thefloating state of the first and second terminals of the first capacitorC1, thus making it possible to stably initialize the first capacitor C1.

Also, during the period (A′), the driving transistor T1 isdiode-connected to compensate for the threshold voltage of the drivingtransistor T1. That is, the driving transistor T1 is diode-connected bythe second transistor T2 in order to apply the threshold voltage Vth ofthe driving transistor T1 between the first and second electrodes of thesecond transistor T2.

During a period (B), the scan signal Sn of the n^(th) scan linemaintains the high signal level, the scan signal Sn−2 of the (n−2)^(th)scan line changes to the low signal level, and the scan signal Sn+2 ofthe (n+3)^(th) scan line and the emission control signal En maintain thelow signal level. Accordingly, the second and fifth transistors T2 andT5 are turned on and the third, fourth and sixth transistors T3, T4 andT6 are turned off.

FIG. 8 illustrates an operation of the pixel circuit in the period (B).

During the period (B), the threshold voltage Vth of the drivingtransistor T1 is compensated. That is, the driving transistor T1 isdiode-connected by the second transistor T2 to compensate for thethreshold voltage Vth of the driving transistor T1. During the period(B), the voltage of the first node N1 is the reference voltage sourceVref, the voltage of the second node N2 is the compensated thresholdvoltage ELVSS+Vto+Vth, and the voltage of the anode electrode of theOLED is the sum (ELVSS+Vto) of the second power supply voltage sourceELVSS and the threshold voltage Vto of the OLED.

According to the embodiment illustrated in FIG. 5, the high-level periodof the scan signal Sn of the n^(th) scan line for compensating thethreshold voltage Vth of the driving transistor T1 may be a duration of3H. Herein, a duration 1H corresponds to 1 row line time (e.g., 1horizontal line time) and 3H corresponds to a duration which is threetimes longer than 1 H. According to other embodiments of the presentinvention, the period for compensating the threshold voltage Vth of thedriving transistor T1 may increase as needed (e.g., from 3H to 4H or5H.) If images are displayed through a high-speed driving operation in alarge-size display panel, the duration 1H may decrease. In this case, asufficient threshold voltage compensation time cannot be secured. Thus,embodiments of the present invention increase the threshold voltagecompensation time corresponding to the period (B), thereby making itpossible to secure the threshold voltage compensation time whenperforming a high-speed driving operation in a large-size display panel.

During a period (C), the scan signal Sn+3 of the (n+3)^(th) scan linechanges to the high signal level, the scan signal Sn of the n^(th) scanline changes to the low signal level, and the scan signal Sn−2 of the(n−2)^(th) scan line and the emission control signal En maintain the lowsignal level. Accordingly, the fourth transistor T4 is turned on and thesecond, third, fifth and sixth transistors T2, T3, T5 and T6 are turnedoff.

FIG. 9 illustrates an operation of the pixel circuit in the period (C).

During the period (C), data can be written. The fourth transistor T4 isturned on to apply the data signal Dm of the current frame, so that thevoltage of the first node N1 would become a data voltage Vdata. As thevoltage of the first node N1 changes from Vref to Vdata, the voltage ofthe second node N2 changes by the voltage variation (Vdata−Vref) of thefirst node N1 through the first capacitor C1. That is, the voltage ofthe second node N2 changes to (ELVSS+Vto+Vth)+(ΔV1). Herein, ΔV1corresponds to the difference (Vdata−Vref) between the data voltageVdata and the voltage of the reference voltage source Vref.

During period (C), the scan signal Sn+3 of the (n+3)^(th) scan linechanges from the high signal level to the low signal level, just priorto the start of a period (D).

During the period (D), the emission control signal En changes to thehigh signal level, and the scan signal Sn−2 of the (n−2)^(th) scan line,the scan signal Sn of the n^(th) scan line, and the scan signal Sn+3 ofthe (n+3)^(th) scan line maintain the low signal level. Accordingly, thethird transistor T3 is turned on and the second, fourth, fifth and sixthtransistors T2, T4, T5 and T6 are turned off.

FIG. 10 illustrates an operation of the pixel circuit in the period (D).

During the period (D), a current flows through the OLED to cause theOLED to emit light. During the period (D), because a driving currentaccording to the voltage level corresponding to the difference betweenthe source voltage and the gate voltage of the driving transistor T1 isgenerated by the driving transistor T1 and the fourth transistor T4 isturned on, an OLED driving current flows through the driving transistorT1 and the OLED. The voltage of the source electrode of the drivingtransistor T1 is equal to the voltage of the anode electrode of theOLED, and the voltage of the anode electrode of the OLED is ELVSS+Voled.Herein, Voled is the voltage applied across the OLED when the OLED emitslight.

The gate voltage of the driving transistor T1 is described below. First,as the voltage of the anode electrode of the OLED changes from(ELVSS+Vto) to (ELVSS+Voled) by the light emission of the OLED, thevoltage of the second node N2 (i.e., a gate voltage Vg of the drivingtransistor T1) changes by the voltage variation (Voled−Vto) of the anodeelectrode of the OLED through the second capacitor C2. Thus, the gatevoltage Vg of the driving transistor T1 changes as Equation (1).Vg=(Vdata−Vref)+(ELVSS+Vto+Vth)+(Voled−Vto)  (1)

Therefore, the gate-source voltage Vgs of the driving transistor T1during the period (D) is expressed as Equation (2).Vgs={(Vdata−Vref)+(ELVSS+Vto+Vth)+(Voled−Vto)}−(ELVSS+Voled)  (2)

The driving current Ioled is determined by the gate-source voltage Vgsas Equations (3) and (4). In Equations (3) and (4), k=β/2, k is aconstant, and β is a gain factor.

$\begin{matrix}\begin{matrix}{{Ioled} = {k\left\lbrack {\begin{Bmatrix}{\left( {{Vdata} - {Vref}} \right) + \left( {{ELVSS} + {Vto} + {Vth}} \right) +} \\{\left( {{Voled} - {Vto}} \right) - \left( {{ELVSS} + {Voled}} \right)}\end{Bmatrix} - {Vth}} \right\rbrack}^{2}} \\{= {k\left\lbrack {\left( {{Vdata} - {Vref} + {Vth}} \right) - {Vth}} \right\rbrack}^{2}}\end{matrix} & (3) \\{{Ioled} = {k\left( {{Vdata} - {Vref}} \right)}^{2}} & (4)\end{matrix}$

Thus, the driving current Ioled outputted from the pixel circuitaccording to an embodiment of the present invention is determinedindependently of the voltage of the first power supply voltage sourceELVDD, the voltage of the second power supply voltage source ELVSS, andthe threshold voltage Vth of the driving voltage Vth. That is, because agray level is implemented according to the difference between Vdata andVref as in Equation (4), a current flows independently of the voltage ofthe second power supply voltage source ELVSS, thereby making it possibleto prevent the degradation of the image quality due to an IR drop. Also,it is possible to display an uniformly bright image independently of thethreshold voltage Vth of the transistor. Also, embodiments of thepresent invention divide the scan signal Sn−2 of the (n−2)^(th) scanline by the initialization signal, thereby securing a sufficientinitialization time in a large-size organic electro-luminescent displayapparatus and thereby improving the contrast ratio.

It can be seen from Equation (4) that the difference between the voltagelevel of the reference voltage source Vref and the data voltage Vdata isimportant for a gray level. For example, if the data voltage Vdata isabout 0˜5V, the voltage of the reference voltage source Vref may beabout 2V. In this case, a black-gradation data voltage Vdata_black isabout 0V (or less than about 2V) and a white-gradation data voltageVdata_white is about 5V. That is, (Vdata_black(0V)=Vref (2V)<Vdata_white(5V).

FIG. 11 illustrates a structure of a pixel circuit according to anotherembodiment of the present invention.

Referring to FIG. 11, the first electrode of the sixth transistor T6 iscoupled to the initial voltage Vinit. Accordingly, during theinitialization period, the sixth transistor T6 applies the initialvoltage Vinit to the second terminal of the first capacitor C1 (the gateelectrode of the driving transistor T1) in response to the scan signalSn−2 of the (n−2)^(th) scan line. Herein, the initial voltage Vinit isgreater than the sum (ELVSS+Vto+Vth) of the second power supply voltagesource ELVSS, the threshold voltage Vto of the OLED, and the thresholdvoltage Vth of the driving transistor T1 (i.e., Vinit=ELVSS+Vto+Vth).

FIG. 12 illustrates a structure of a pixel circuit according to anotherembodiment of the present invention.

Referring to FIG. 12, the first electrode of the sixth transistor T6 iscoupled to the reference voltage source Vref. Accordingly, during theinitialization period, the sixth transistor T6 applies the voltage ofthe reference voltage source Vref to the second terminal of the firstcapacitor C1 (the gate electrode of the driving transistor T1) inresponse to the scan signal Sn−2 of the (n−2)^(th) scan line. Theembodiment illustrated in FIG. 12 reduces the number of interconnectionsrelative to the embodiment illustrated in FIG. 11.

FIG. 13 is a flow chart illustrating a method of driving an organicelectro-luminescent display apparatus according to an embodiment of thepresent invention.

Referring to FIG. 13, a first scan signal corresponds to the scan signalSn−2 of the (n−2)^(th) scan line, a second scan signal corresponds tothe scan signal Sn of the n^(th) scan line and a third scan signalcorresponds to the scan signal Sn+3 of the (n+3)^(th) scan line.

Operation S101 is an initialization period. When the first scan signalSn−2 (i.e., the initialization signal) has a high signal level, thesixth transistor T6 is turned on to initialize the second node N2 towhich the second terminal of the first capacitor C1 and the gateelectrode of the driving transistor T1 are coupled. Herein, theinitialization voltage of the second node N2 may be the voltage level ofthe first power supply voltage source ELVDD, the initial voltage Vinit,or the voltage level of the reference voltage source Vref according tothe voltage source coupled to the first electrode of the drivingtransistor T6.

Operation S102 is a threshold voltage compensation period. When thesecond scan signal Sn has a high signal level, the second transistor T2is turned on to diode-connect the driving transistor T1.

The initialization period of operation S101 and the threshold voltagecompensation period of operation S102 may partially overlap with eachother. In operation S102, the fifth transistor T5 is turned on to fixthe voltage of the first node N1 to the voltage level of the referencevoltage source Vref. If it is performed concurrently (e.g.,simultaneously) with the initialization of the second node N2 inoperation S101, then the first capacitor C1 can be stably initializedwithout becoming a floating state. Also, the period of operation S102may increase if necessary for a high-speed driving operation of alarge-size display panel. Accordingly, a sufficient threshold voltagecompensation period can be secured.

Operation S103 is a data write period. When the third scan signal Sn+3has the high signal level, the data voltage Vdata is applied to thefirst node N1 and the voltage corresponding to the data voltage Vdata isstored in the first capacitor C1.

Operation S104 is an OLED light emission period. When the emissioncontrol signal has the high signal level, the driving current Ioled isoutputted to the anode electrode of the OLED. As expressed in Equation(4), the level of the driving current Ioled is determined according tothe voltage level Vdata of the data signal Dm stored in the firstcapacitor C1. The OLED emits light, the brightness of which correspondsto the level of the driving current Ioled.

According to the embodiments of the present invention, the drivingcurrent outputted from the OLED can be determined independently of thethreshold voltage of the driving transistor and the cathode power supplyvoltage of the OLED. Accordingly, it is possible to eliminate thethreshold voltage variation of the driving transistor and the IR dropcaused by a parasitic resistance of the interconnection transferring thecathode power supply voltage of the OLED. Additionally, the contrastratio in the large-size display panel can be improved by dividing theinitialization period, and a suitable high-speed driving operation isprovided by controlling the compensation time of the threshold voltage.

While exemplary embodiments of the present invention have beenparticularly shown and described, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thepresent invention as defined by the following claims.

What is claimed is:
 1. A pixel circuit comprising: a light emittingdevice comprising a first electrode and a second electrode; a drivingtransistor comprising: a first electrode; a second electrode; and a gateelectrode, the driving transistor for outputting a driving currentaccording to a voltage applied to the gate electrode of the drivingtransistor; a first capacitor comprising a first terminal and a secondterminal, the second terminal of the first capacitor being coupled tothe gate electrode of the driving transistor; a second capacitorcomprising a first terminal coupled to the gate electrode of the drivingtransistor and a second terminal directly coupled to the first electrodeof the light emitting device, the second capacitor being different fromthe first capacitor; a second transistor comprising: a first electrodecoupled to the gate electrode of the driving transistor; a secondelectrode coupled to the first electrode of the driving transistor; anda gate electrode, wherein the second transistor is configured to beturned on in response to a second scan signal applied to the gateelectrode; a third transistor for applying a first power supply voltageto the first electrode of the driving transistor in response to anemission control signal, the third transistor comprising a firstelectrode directly connected to a first power supply voltage line; afourth transistor comprising a first electrode for receiving a datasignal and a second electrode coupled to the first terminal of the firstcapacitor, wherein the fourth transistor is configured to be turned onin response to a third scan signal applied to the gate electrode; afifth transistor comprising: a first electrode directly connected to areference voltage line for receiving a reference voltage; a secondelectrode coupled to the first terminal of the first capacitor; and agate electrode, wherein the fifth transistor is configured to be turnedon in response to the second scan signal applied to the gate electrode;and a sixth transistor comprising: a first electrode for receiving afirst voltage; a second electrode directly electrically connected to thegate electrode of the driving transistor; and a gate electrode, whereinthe sixth transistor is configured to be turned on in response to afirst scan signal applied to the gate electrode, wherein the drivingtransistor and the second to sixth transistors are N-type transistors,and wherein the second electrode of the driving transistor is directlyconnected to the light emitting device.
 2. The pixel circuit of claim 1,wherein the sixth transistor is configured to apply the first voltage tothe gate electrode of the driving transistor in response to the firstscan signal applied to the gate electrode of the sixth transistor. 3.The pixel circuit of claim 2, wherein the fifth transistor is configuredto apply the reference voltage to the first terminal of the firstcapacitor in response to the second scan signal applied to the gateelectrode of the fifth transistor, and the second transistor isconfigured to diode-connect the driving transistor in response to thesecond scan signal applied to the gate electrode of the secondtransistor.
 4. The pixel circuit of claim 3, wherein the second scansignal is a scan signal of an n^(th) period.
 5. The pixel circuit ofclaim 4, wherein the first scan signal and the second scan signal areboth maintained at a transistor turn-on level during an overlap period,the transistor turn-on level being a voltage level configured to turn-onthe second transistor when the voltage level is applied to the gateelectrode of the second transistor.
 6. The pixel circuit of claim 5,wherein the second scan signal is maintained at a high signal level fora duration greater than the overlap period.
 7. The pixel circuit ofclaim 2, wherein the first scan signal is a scan signal of an (n−2)^(th)period.
 8. The pixel circuit of claim 1, wherein the fourth transistortransfers the data signal to the first terminal of the first capacitorin response to the third scan signal applied to the gate electrode ofthe fourth transistor.
 9. The pixel circuit of claim 8, wherein thethird scan signal is a scan signal of an (n+3)^(th) period.
 10. Thepixel circuit of claim 1, wherein the light emitting device is anorganic light emitting diode (OLED).
 11. The pixel circuit of claim 1,wherein the driving transistor and the second to sixth transistors areN-type metal-oxide semiconductor field effect transistors (MOSFETs). 12.The pixel circuit of claim 1, wherein the first electrode of the drivingtransistor is a drain electrode and the second electrode of the drivingtransistor is a source electrode.
 13. The pixel circuit of claim 1,wherein the first voltage applied to the first electrode of the sixthtransistor is the first power supply voltage.
 14. The pixel circuit ofclaim 1, wherein the first voltage applied to the first electrode of thesixth transistor is an initial voltage.
 15. The pixel circuit of claim1, wherein the first voltage applied to the first electrode of the sixthtransistor is the reference voltage.
 16. A method of driving a pixelcircuit as recited in claim 1, the method comprising: initializing thegate electrode of the driving transistor when the sixth transistor isturned on in a first period; initializing the first terminal and thesecond terminal of the first capacitor when the sixth transistor and thefifth transistor are turned on in a second period; compensating for athreshold voltage of the driving transistor when the fifth transistorand the second transistor are turned on in a third period; applying thedata signal when the fourth transistor is turned on in a fourth period;and applying the first power supply voltage to the driving transistorand generating a driving voltage so that the light emitting device mayemit light when the third transistor is turned on in a fifth period. 17.An organic electro-luminescent display apparatus comprising: a scandriver for supplying a scan signal to a plurality of scan lines; anemission control driver for supplying an emission control signal to aplurality of emission control lines; a data driver for supplying a datasignal to a plurality of data lines; and a plurality of pixel circuitslocated at crossing regions of the scan lines, the emission controllines and the data lines, wherein each pixel circuit of the plurality ofpixel circuits comprises: an organic light emitting diode (OLED)comprising a first electrode and a second electrode; a drivingtransistor comprising: a first electrode; a second electrode; and a gateelectrode, the driving transistor for outputting a driving currentaccording to a voltage applied to the gate electrode of the drivingtransistor; a first capacitor comprising a first terminal and a secondterminal coupled to the gate electrode of the driving transistor; asecond capacitor comprising a first terminal coupled to the gateelectrode of the driving transistor and a second terminal directlycoupled to the first electrode of the OLED, the second capacitor beingdifferent from the first capacitor; a second transistor comprising: afirst electrode coupled to the gate electrode of the driving transistor;a second electrode coupled to the first electrode of the drivingtransistor; and a gate electrode, wherein the second transistor isconfigured to be turned on in response to a second scan signal appliedto the gate electrode; a third transistor for applying a first powersupply voltage to the first electrode of the driving transistor inresponse to the emission control signal, the third transistor comprisinga first electrode directly connected to a first power supply voltageline; a fourth transistor comprising: a first electrode coupled to thedata signal, a second electrode coupled to the first terminal of thefirst capacitor; and a gate electrode, wherein the fourth transistor isconfigured to be turned on in response to a third scan signal applied tothe gate electrode; a fifth transistor comprising: a first electrodedirectly connected to a reference voltage line coupled to a referencevoltage; a second electrode coupled to the first terminal of the firstcapacitor; and a gate electrode, wherein the fifth transistor isconfigured to be turned on in response to the second scan signal appliedto the gate electrode; and a sixth transistor comprising: a firstelectrode coupled to the first power supply voltage; a second electrodedirectly electrically connected to the gate electrode of the drivingtransistor; and a gate electrode, wherein the sixth transistor isconfigured to be turned on in response to a first scan signal applied tothe gate electrode, wherein the driving transistor and the second tosixth transistors are N-type transistors, and wherein the secondelectrode of the driving transistor is directly connected to the lightemitting device.
 18. The organic electro-luminescent display apparatusof claim 17, wherein the gate electrode of the sixth transistor iscoupled to an (n−2)^(th) scan line of the scan lines and is configuredto apply the first power supply voltage to the gate electrode of thedriving transistor in response to the scan signal applied to the gateelectrode of the sixth transistor.
 19. The organic electro-luminescentdisplay apparatus of claim 17, wherein the gate electrode of the fifthtransistor is coupled to an n^(th) scan line of the scan lines and isconfigured to apply the reference voltage to the first terminal of thefirst capacitor in response to the scan signal applied to the gateelectrode of the fifth transistor.
 20. The organic electro-luminescentdisplay apparatus of claim 17, wherein the gate electrode of the secondtransistor is coupled to an n^(th) scan line of the scan lines and isconfigured to diode-connect the driving transistor in response to thescan signal applied to the gate electrode of the second transistor. 21.The organic electro-luminescent display apparatus of claim 17, whereinthe gate electrode of the fourth transistor is coupled to an (n+3)^(th)scan line of the scan lines and is configured to transfer the datasignal to the first terminal of the first capacitor in response to thescan signal applied to the gate electrode of the fourth transistor.